Systems and methods herein generally relate to current leakage in circuitry and to correcting for current leakage in circuits that are used in humid environments.
For some high-voltage power supply applications, current leakage from the high-voltage output circuit should be limited because the amount of current leakage correspondingly reduces the amount of output current. One example of such an application is a high-voltage DC current source for charging a bias transfer roll (BTR) in an electrostatic printer. Typically, voltages of up to 6 kV are required in a bias transfer roll to obtain currents well below 100 uA. The current source in a bias transfer roll must also have an accuracy of uA's, so any current leakage can substantially decrease the performance of the bias transfer roll.
High-voltage circuits on printed circuit boards (PCB's) can easily leak current if they are situated in a humid environment. The PCB material has the ability to absorb water, which makes the surface relatively conductive, resulting in current leakage. Factors that contribute to current leakage are the material properties and the level of contamination of the surface (flux residue) of the PCB. Further, this effect is especially noticeable at high voltages and low currents, where operation relies on the insulating properties.
There are a number of methods used to prevent or reduce such current leakage. One method is to encapsulate the high-voltage circuit in a housing so that it is not exposed to the humid environment. These (potting) techniques are very effective, but relatively costly. If the high-voltage output must be available on the PCB for interfacing, this technique is not adequate.
Another method is to use shielding techniques, where the high-voltage (circuits) are surrounded or separated by conductive shields. The leakage current, picked-up by the shield, is measured and used for correcting the output current or voltage accordingly. This works best if the shield is nearby the high-voltage circuit and completely surrounds it. However, such shields increase the leakage and can potentially cause insulation breakdown (arcing/tracking). Also, on single-sided PCB's the leakage across the top side surface (where there is no shield) cannot be measured.
Another method, commonly used in printing machines, is to utilize a humidity sensor. With this, the relative humidity of the general machine environment is monitored and fed back to the processor unit for changing the setpoints required for maintaining the image quality. One of these setpoints is the BTR current. This approach does not take into account that the (relative) humidity in the area where the high-voltage power supply (HVPS) resides can be completely different. Also, the contamination degree of the HVPS PCB surface and the content of previously absorbed water in the PCB is not a constant factor over time. Furthermore, such methods rely on an HVPS manufactured with controlled PCB cleanliness.